Inhibin is a peptide hormone produced by the gonads which modulates the secretion of follicle stimulating hormone (FSH). It has been shown to be present in steroid-free extracts of ovine rete testis fluid in porcine follicular fluid, bovine seminal plasma and ovine testicular lymph. A sensitive bioassay for inhibin has been developed using dispersed anterior pituitary cells from rats in short-term culture. Materials which contain inhibin have been shown to cause a dose-related suppression of gonadotropin releasing hormone (GnRH) stimulated FSH secretion by these cell. Ovine testicular lymph protein (OTLP) has been used as a standard in this assay. RTF, which is a rich source of inhibin activity, is collected from rams and is used as the starting material for purification. The proteins are precipitated from RTF by ammonium sulphate, gel filtered and subjected to ion exchange chromatography in reasonable yield and with increased specific activity. These are monitored by the bioassay. It is planned to purify this material further using isoelectric focussing, polyacrylamide gel electrophoresis and immunoaffinity chromatography. Purity will be checked by N-terminal end group analysis and the ultimate aim will be to determine the amino acid sequence of inhibin, which will only be possible if inhibin proves to be a relatively small molecule. Once inhibin has been purified a radioimmunoassay will be developed and physiological studies undertaken to determine the relationship between inhibin and the secretion of FSH.